This invention relates to a device for the cryogenic transfer of specimens. The invention is also concerned with apparatus for the cryogenic treatment of specimens, more particularly with apparatus for the cryofixation and/or cryopreparation of specimens.
In known apparatus for cryofixation or for cryopreparation, liquid nitrogen is generally used as a coolant and is received in a cooling chamber of the apparatus for this purpose. The cryofixation of specimens in the form of biomedical test samples or similarly conditioned physico-chemical test samples is usually carried out using one of two methods. In one method the specimens are rapidly cooled by insertion into a liquid cooled close to its freezing point by the coolant; the liquid may be, for example, propane, ethane or a halogenohydrocarbon. In the other method the specimens are cooled by application to a metal surface of a metal block cooled by the coolant.
The liquid nitrogen used for cooling the liquid or the metal block is generally located beneath the liquid or beneath the metal block.
After cryofixation of the specimen it is usually placed in a specimen holder for cryotransfer to the next stage in its treatment. The apparatus for the subsequent stages such as a cryoultramicrotomy, freeze fracture, freeze drying, cryosubstitution and cryoelectron microscopy, is separate from the cryofixation apparatus. During the cryotransfer step the specimen must be maintained at a temperature in or below the range -160.degree. C. to -100.degree. C., because the structure of the specimen or the distribution of the individual components would change at temperatures above this range. In order to transfer without risk the specimen holder to the apparatus in the next stage of the treatment it is usual to fill the specimen holder with liquid nitrogen so that the cryotransfer of the specimen holder to the next apparatus is conducted with the specimen immersed in liquid nitrogen.
It is common for the liquid nitrogen in the cooling chamber to be covered by a cover comprising a net or sieve plate, the meshes or openings of which are dimensioned in such a way that the specimen and the specimen holder cannot pass through the cover. This prevents specimens from being accidently dropped in the liquid nitrogen from where the samples would be difficult to retrieve, and could be lost. The provision of the cover has the disadvantage that the liquid nitrogen in the cooling chamber cannot be easily transferred from the chamber to the specimen holder.
In view of this the liquid nitrogen to be put in the specimen holder prior to cryotransfer has to be obtained from another source. In practice the only other available source is from a liquid nitrogen supply container. This is not an easy operation because the amount of liquid nitrogen required for cryotransfer is about 5 ml, while the supply containers usually contain 20 to 100 litres and are not adapted to dispense small quantities of liquid nitrogen. Obtaining such small quantities of liquid nitrogen from the supply containers results in a considerable amount of wastage of the liquid nitrogen, and is not without risk to the technician performing the operation. This is an important problem because during the daily laboratory routine a large number of such cryotransfer operations have to be performed, often more than 10 per hour.